When do we say: good to and when good for. Well, when do we say "to" and when "for". Is it any rule?
Eg. "it makes no sense TO me". what about "you wrote your level test. you got 90%. is it good TO/FOR you?"

Please, help Emotion: smile
Context matters.

Johnny is so good to his mother = Johnny is considerate and spend a lot of time doing things for his mother

Johnny is so good for his mother = Johnny's actions benefit his mother who is therefore healthier and happier.

In your example "Is it good for you" means Is this mark one that is normal for you or exceptional for you to have achieved?. It is a matter of relation to you.
Thank you Emotion: smile